US Patent Application No. 2008/0239491 discloses a polymer film having positive birefringence greater than 0.002 throughout the wavelength range of 400 nm<λ<800 nm, wherein the film having been cast onto a substrate from a solution of a polymer having a moiety of
wherein R1, R2, and R3 are each independently hydrogen atoms, alkyl groups, substituted alkyl groups, or halogens, wherein OASU is a disk-like group or a mesogen, and wherein OASU is attached to the polymer backbone through a single covalent bond.
The most common polymer having a disk-like OASU is polystyrene, the solution-cast film of which generally has a birefringence of 0.001-0.002. It was disclosed in the '491 application that the birefringence of polystyrene could be increased by incorporating a birefringence-enhancing substituent (BES) such as bromo or nitro group onto the benzene ring. For example, poly(nitrostyrene) was reported to have a birefringence as high as about 0.016, while poly(bromostyrene) as high as about 0.007.
In contrast, the present inventors have unexpectedly discovered that the birefringence of the polystyrene film can be greatly increased by incorporating fluorine atoms onto the backbone of the polystyrene molecule. Such a polymer film has a birefringence as high as about 0.015-0.02. Moreover, the polymer film exhibits much improved mechanical properties over the polystyrene film. The high birefringence enables the casting of thin films onto a substrate to yield a compensation film having a desirable retardation, while the improved mechanical properties allow the preparation of free standing films with high strength, which can be stretched, rolled, and laminated.